This invention relates to a MISFET switching circuit having a high withstand voltage.
In general, the gate-drain voltage of an insulated gate field-effect transistor (MISFET) at the time when the drain junction breaks down is as low as several tens of volts (for example, 20 V). For this reason, a high supply voltage cannot be used for a circuit usually constructed of MISFETs, such as a circuit for an electronic desk top calculator, and the use of the MISFET has been restricted. In, for example, a display element driver circuit for indicating a calculated result etc., the driving voltage of a display element is 25 V - 50 V and is somewhat higher than the withstand voltage of the MISFET, and hence, the display element cannot be directly driven by the MISFET. The driver circuit has therefore been constructed of bipolar transistors etc. In the electronic desk top calculator, accordingly, the display element driver circuit and a control circuit cannot be put into an IC (integrated circuit) on an identical chip. This poses a problem for the fabricating process, cost reduction, etc.
"In order to solve the problem, a circuit arrangement as described in Japanese patent application No. 5971/1973, laid open as Laid Open Print No. 95561/1974 to public inspection in Japan on Sept. 10, 1974 has been proposed." FIG. 1 of the accompanying drawing shows the digitron driver circuit proposed.
Referring to the figure, Q.sub.A designates an enhancement mode driving MISFET, while Q.sub.B denotes an enhancement mode protecting MISFET. The MISFET Q.sub.A has its source terminal grounded, and has its drain terminal connected to the source terminal of the MISFET Q.sub.B.
The drain terminal of the MISFET Q.sub.B is connected through a load resistance R to a bias supply voltage V.sub.DD. An output is derived from the drain terminal of the MISFET Q.sub.B, to drive a digitron D.A control input signal V.sub.in is applied to the gate terminal of the MISFET Q.sub.A, while a d.c. voltage V.sub.GG is applied to the gate terminal of the MISFET Q.sub.B.
In the driver circuit, the maximum value of the bias supply voltage as applied to the drain terminal of the MISFET Q.sub.B when the MISFET Q.sub.A is in the "off" state becomes .vertline.BV.sub.2 .vertline. - .vertline.V.sub.GG - V.sub.thE .vertline.. This voltage is the digitron driving voltage.